The disclosure relates to a preferably electro-magnetically actuated valve and in particular to a low-pressure valve, as used, for example, in a hydraulic machine.
Valve-controlled hydraulic machines of this type are known, for example, from EP 1 537 333 B1. The European patent document shows a hydraulic machine of axial or radial piston construction which can be operated in principle as a motor or as a pump, with the volumetric delivery or capacity being adjustable via the valve timing gear. In an exemplary embodiment which is described, the hydraulic machine is embodied in the form of an axial piston machine, wherein a multiplicity of pistons arranged in a cylinder is supported on a rotatably mounted swash plate. Each piston together with the associated cylinder space delimits a working space which can be connected via a valve on the low-pressure side and a valve on the high-pressure side to a pressure medium inlet or to a pressure medium outlet.
In the known solution, the two valves are embodied in the form of electrically releasable or lockable nonreturn valves which are actuable via the pump timing gear in order to operate the particular working space in “full mode”, in “partial mode” or in “idle mode”. As a result, the volumetric delivery or capacity can be adjusted in an infinitely variable manner from a maximum value to 0. The hydraulic machine is operated in accordance with a regulating algorithm via a control unit in order to obtain a total volumetric delivery flow (pump) or total capacity flow (motor) with as few pulsations as possible. The volumetric flow is frequently adjusted by a phase-gating control, but may also be adjusted by a phase-chopping control.
Hydraulic machines with a capacity/volumetric delivery which can be changed via the valve timing gear are also referred to as digital displacement units (DDU). All positive displacement principles are basically applicable in this case. However, piston machines, in particular of radial piston construction, are advantageous, since said piston machines make it possible to separately form and therefore actively control the input and output for each positive displacer. In this case, it may be highly expedient to differentiate between pump and motor operation such that then the control element may differ in appearance for the low-pressure and high-pressure connections.
A prerequisite for the above-described type of control (DDU) is that the valves on the low-pressure and high-pressure sides can be switched in highly dynamic fashion such that the above-described pressure medium flow paths can be very rapidly blocked off or opened up for flow. The control elements on the low-pressure side or high-pressure side can be embodied in the form of, for example, switching valves, preferably of seat-type construction, which are preferably actuable by a magnetic actuator. Various and sometimes contradictory requirements are imposed on a valve of this type. A hydraulic machine which is minimized in respect of construction space requires a valve, the overall dimensions of which are small and which has a flow cross section with is as large as possible and is therefore low in resistance. However, such a large flow cross section requires a greater valve lift, this contradicting the requirement for high valve dynamics with as little electrical power consumption as possible. Furthermore, the requirements imposed on the valve vary at different operating points of the hydraulic machine. For example, large volumetric flows and requirements for low switching times arise from high rotational speeds, and low rotational speeds are associated with long switching-on periods for the magnet coils. The mechanical loads and requirements imposed on the sealing system also change via the pressure prevailing at the particular connection.
U.S. Pat. No. 7,077,378 B2 discloses a valve on the low-pressure side for a hydraulic machine of this type, wherein an annular throughflow opening is closed by an approximately cup-shaped valve element. Said annular throughflow opening is bounded by an inner and an outer sealing seat, and therefore the specific surface pressure on the sealing edge is comparatively low compared to a conventional valve cone. In the known solution, the platelike or cup-shaped valve element is prestressed into an open position via a spring and can be adjusted by means of a magnetic actuator into its closed position in which the valve element rests on the above-described sealing seats and the throughflow opening is blocked. During flow through said valve, the platelike valve element, which is prestressed in the opening direction thereof, can be acted upon by flow forces effective in the closing direction, and therefore the throughflow cross section is reduced and, correspondingly, the pressure loss is increased. Although said undesirable closing movement could be countered by a more powerful opening spring, the valve dynamics would deteriorate as a result or a more powerful magnetic coil together with associated power electronics would be necessary. To avoid this drawback, use is made, according to U.S. Pat. No. 7,077,378 B2, of a permanent magnet which acts upon the valve element in the open position thereof with a magnetic force. When the magnetic actuator is energized, the field of the permanent magnet is neutralized and, in addition, a magnetic force which is effective in the closing direction is generated. With a solution of this type, a correspondingly more efficient magnetic actuator is therefore required. In addition, this solution does not exhibit the desired dynamics either, since the field of the permanent magnet has to be weakened first before the valve element can be moved in the direction of the closed position thereof. Furthermore, a relatively high contact pressure force is required between the valve element and valve seat in order to ensure adequate tightness of the valve in the closed state.